1. Field
The present invention relates to a complementary metal oxide semiconductor (CMOS) image sensor, and more particularly, to a CMOS image sensor having a great photosensitivity and a method for fabricating the same.
2. Description of the Background Art
A charge-coupled device (CCD) image sensor has been much used owing to its simple circuit and its good performance about a picture quality and a noise. However, because a mobile equipment needs a low power and a high integration, attention is being paid to a complementary metal oxide semiconductor (CMOS) image sensor satisfying the low power and the high integration.
The CMOS image sensor refers to a semiconductor device for converting an optical image into an electric signal. The CMOS image sensor comprises a light detecting part for detecting light, and a logic circuit part for data processing the detected light into the electric signal. The CMOS image sensor employs a switching method in which a MOS transistor is provided as much as the number of pixels using a CMOS technology, and an output is sequentially detected using the MOS transistor.
The CMOS image sensor is fabricated using a conventional CMOS transistor process. The CMOS image sensor has an advantage of less power consumption, low price, and high integration. Accordingly, by an intensive research for some years past, the CMOS image sensor is being expected to be an alternative of the CCD image sensor in many applications.
However, despite having the lower power and the high integration, the CMOS image sensor has a disadvantage that it has a smaller photosensitivity than the conventional CCD image sensor due to a dark current. Thus, many researches are in progress to solve the disadvantage.
The conventional CMOS image sensor will be described below.
FIG. 1 is a plan view illustrating the conventional CMOS image sensor. FIG. 2 is a cross-sectional view taken along line 2-2′ of FIG. 1.
Only a light receiving region for receiving light, and a transistor region for processing the received light into an electric signal will be illustrated and described below.
In the conventional CMOS image sensor, light receiving elements 106 and 108 such as a photodiode (PD) are formed in the light receiving region on a silicon substrate 101 having P-impurity regions 102 formed at both sides. A floating diffusion 104 is formed in parallel at one side of the light receiving elements 106 and 108 in the transistor region. A transfer gate 103 is positioned on the floating diffusion 104. A shallow trench isolation 105 is formed along an edge of the silicon substrate 101 to isolate the above devices from other devices.
The CMOS image sensor comprises a reset transistor (Rx), a driving transistor (Dx), and a selection transistor (Sx) connecting with the floating diffusion 104.
A non-described reference numeral 107 denotes an oxide film.
As shown in FIG. 2, in the conventional CMOS image sensor, the light receiving elements 106 and 108 are formed by a large area to effectively receive a large amount of light. The light receiving elements 106 and 108 are of a structure where a P+ impurity region 106 doped with P-type impurities is layered on an N impurity region 108 doped with N-type impurities, and are of a type where they are exposed as light receiving surfaces.
The light receiving surfaces of the light receiving elements 106 and 108 are of a flat structure where they are formed of crystalline silicon. Due to the flat structure, the conventional light receiving elements 106 and 108 have a limitation of a light receiving capability as follows.
First, in light receiving portions of the light receiving elements 106 and 108, the crystalline silicon has a small absorption coefficient for visible rays. In general, a wavelength of light absorbed by material is greatly dependent on a bandgap of the material. The crystalline silicon used for the light receiving portions of the light receiving elements 106 and 108 has an indirect bandgap of 1.12 eV and thus, absorbs only light having a short wavelength of 1.1 μm or less. However, the absorption coefficient of the crystalline silicon for the visible rays of the wavelength of 1.1 μm or less cannot sufficiently generate carriers transferred to the floating diffusion 104. Thus, in the crystalline silicon of the light receiving portion, its absorption coefficient is too small to sufficiently form the carriers.
Second, there is a loss of light not absorbed but reflected from the crystalline silicon that is the light receiving portion. Some of the light incident on the light receiving elements 106 and 108 induce light generation and form the carriers flowing to the floating diffusion 104, but other is reflected and lost without re-absorption. It is known in the art that an amount of the reflected and lost light is greater than an amount of the light inducing the light generation.
Third, a light receiving area of the CMOS image sensor does not exceed an area of the CMOS image sensor. In the conventional CMOS image sensor, the light receiving elements 106 and 108 are parts of the CMOS image sensor. Thus, the area of the light receiving elements 106 and 108 does not exceed the area of the CMOS image sensor. Since a cell size of the CMOS image sensor should reduce for high integration, a reduction of the area of the light receiving elements 106 and 108 gradually degrades the light receiving capability of the CMOS image sensor.